There may be a requirement for a shared mailbox to send automatic replies. This is possible for a member of the shard mailbox to set themselves, without having to get an administrator involved.
See below instructions needed to be followed by a member of the shared mailbox in question:
1. Log into the Microsoft web apps by navigating to portal.office.com and signing in
2. On the list of apps on the left side of the screen, select the outlook icon.
3. Once Outlook web ap has loaded, click on your user profile icon/initials in the top right hand corner of the screen, and select 'Open Another Mailbox'
4. Type in the email address of the shared mailbox in question and click 'open'
5. Once this mailbox has opened, click on the settings cog in the top right, and select 'All Outlook Settings'
6. Click on the 'Automatic replies' section, and set the requested message.
It is worth setting these auto replies to be set only within the requested time period, so users don't have to repeat this process in order to turn the auto replies off. This option is available within the 'Automatic replies' section as referred to above.